The present invention relates to separating nitrogen from methane in the production of liquefied natural gas (xe2x80x9cLNGxe2x80x9d).
U.S. Pat. No. 6,070,429 to Low et al., the disclosure of which is incorporated herein by reference, describes a process for removing nitrogen from natural gas and other methane-containing gases during the production of LNG. In this process, a nitrogen recovery unit (xe2x80x9cNRUxe2x80x9d) composed of three separate multistage stripping towers is used to recover a high purity nitrogen stream (stream 438) which can be vented to the atmosphere. Also produced are two mixed nitrogen/methane streams, one containing about 10% nitrogen (stream 440) and the other containing about 2.8% nitrogen (stream 436), which are recycled to the open methane cycle gas stream. Because of this recycle, extra horsepower must be expended in operating the cascaded refrigeration system of the plant due to the nitrogen content of these recycled streams.
In accordance with the present invention, it has been found that substantially all the nitrogen can be removed from natural gas during the production of LNG, without producing mixed nitrogen/methane streams needing recycle and further processing, by operating both the high pressure and the low pressure multistage distillation towers of a two column cryogenic nitrogen rejection unit to produce acceptable natural gas liquids as tower bottom products, while the low pressure tower is further operated to produce as an overhead a nitrogen gas steam preferably containing no more than about 1% methane for safe venting to the atmosphere.
Thus, the present invention provides a process for removing nitrogen from a methane-containing feed gas during the production of a liquefied natural gas product using a two column cryogenic nitrogen recovery unit having a high pressure multistage distillation tower and a low pressure multistage distillation tower, the process comprising separating the feed gas in the high pressure multistage distillation tower into a first methane-rich liquid bottoms containing a reduced nitrogen content and a first vaporous overhead, at least partially condensing the first vaporous overhead into a liquid intermediate stream, separating the liquid intermediate stream in the low pressure multistage distillation tower into a second methane rich bottoms containing a reduced nitrogen content and a second vaporous overhead containing a substantial portion of the nitrogen in the feed gas and a substantially reduced methane content, and recovering the first methane-rich liquid bottoms and the second methane-rich liquid bottoms as the liquid natural gas product of the process.